A varactor diode is a semiconductor device having voltage sensitive capacitance. It operates as a typical reverse biased diode. The capacitance of the device is primarily a function of the junction cross-sectional area and the length of the depletion region at the junction. The applied voltage (bias) determines the length of the depletion region and thus the capacitance.
The depletion region length is also a function of semiconductor doping. In an ideal diode, dopant concentration and distribution is fixed. However, in real diodes, dopant concentration and distribution is affected by surface states which varies the charge configuration of the diode. In addition, surface states are unstable and fluctuate with time. Thus, surface states are said to be "noisy".
Since surface states modify the dopant profile, the depletion region length, and therefore diode capacitance, is affected. The time-dependence of surface states introduce the same dependence into the depletion region length and related diode capacitance. Thus, the diode capacitance becomes noisy.
Referring now to FIG. 1, for a given reverse bias, varactor 10 may be modeled as a fixed capacitor 12 in parallel with a small variable capacitor 14 having randomly changing capacitance (mean=0, variance=.sigma..sup.2). Thus, EQU .vertline.C.sub.14 .vertline.&lt;&lt;C.sub.12
Obviously, in tuned circuits using varactor diode tuning C.sub.14 modulates the resonant frequency. In oscillator designs, C.sub.14 is often the dominant phase noise mechanism at off-set frequencies close to the carrier.
The present invention employs an array of several--even many--varactors. Other, well known applications have used several varactors, but not in the same way or for the same reason. Where multiple, series-connected varactors are used in voltage controlled oscilator (VCO) circuits for preventing breakdown, the RF voltage across any one diode is reduced and reduces the likelihood of failure of a single varactor. Multiple varactors are also used to lower frequency modulation distortion; for example, odd-order frequency modulation distortion can be reduced by using a number of varactors in series. Finally, several varactors are often connected in parallel in a given circuit to increase capacitance without affecting other characteristics associated with a single varactor in the same circuit. None of these prior uses are shown nor suggested to improve the inherent noise characteristics of the circuit.
Prior to development of the present invention, varactor-induced noise in a VCO was tolerated, reduced by decoupling the varactors from the circuit or employing a phase-lock loop. Decoupling the varactors decreases the oscillator tuning range and, depending on the application, may force the designer to use several VCO's where only one is actually necessary. If a phase-lock loop is employed, VCO tuning is also impaired and substantial additional circuitry is required.
In accordance with the principles of the present invention, in any application sensitive to varactor noise, an array of varactor diodes may be used in place of the single, otherwise adequate, varactor. The diodes are arranged in series-connected blocks of parallel-connected diodes. Obviously, the capacitance and biasing of each diode can be computed to provide the equivalent capacitance required for the particular application. However, an improvement in noise performance approximately equivalent to the square root of the number of diodes used in the array is achieved.